An electroluminescence (“EL”) device is a device which makes use of the phenomenon of electro luminescence to emit light. An EL device generally includes thin film transistors (“TFT”) and a light-emitting diode (“LED”) further including a light-emitting layer. If the light-emitting layer contains organic light-emitting material, the device is referred to as an organic EL device. When a current passes between a cathode and an anode of the LED device, light is emitted through the light-emitting layer.
Typically, EL devices may be classified into voltage-driven and current-driven types. As compared to a current-driven EL device, a voltage-driven EL device may be disadvantageous in non-uniform pixel brightness caused by different threshold voltages and mobility of TFTs. Examples of current-driven EL devices are found in U.S. Pat. No. 6,373,454 to Knapp, entitled “Active Matrix Electroluminescence Devices, and U.S. Pat. No. 6,501,466 to Yamagishi, entitled “Active Matrix Type Display Apparatus and Drive Circuit Thereof.”
For current-driven EL devices, the magnitude of a current flowing through an LED determines the gray scale or brightness of a pixel. As an example of a 6-bit gray-scale spectrum, if the maximum gray-scale current is 2.5 μA (microampere), the minimum gray-scale current is approximately 0.04 (=2.5/[26−1]) μA. Data line loading, that is, the resistance and parasitic capacitance of a data line corresponding to the pixel, may adversely affect smaller gray-scale currents. For example, if the parasitic capacitor is not charged or discharged to a stable state when a corresponding scan line is turned off, a poor pixel display may result. It is thus desirable to have a current-driven EL device that provides reliable gray-scale display and uniform brightness, especially in smaller gray-scale regions.